[00:03] Two face flash on the screen. A street with a street lamp is out of focus. Text in the corner reads:

YOU ABANDONED HIM

[00:11] A person is visibly out of focus walking towards camera. Text overlaps:

I SAW

[00:15] Someone is running through what appears to be the forest of Rosswood Park. Panting can be heard. Text reads:

HE IS SEARCHING

[00:18] The words ALEX and FOR YOU are barely visible for a second
[00:21] It cuts to the street again. The person is closer.
[00:26] For a split second a red screen appears with the pixelated Marble Hornets logo
[00:27] Camera cuts to what appears to be the tunnel flooded with water. It slowly fades
[00:34] Binary numbers are shown. Some letters are in the numbers. The letters spell out:

WATCH DEATH

Main Theories and Observations

There is footage of two faces overlaid on this entry:

One at 0:03 that is ambiguous but shares some features with Tim.

One at 0:26 which shares some features with Jessica.

Ten letters can be found in the screen full of zeroes at 0:34

Unscrambled, they most likely spell WATCH DEATH.

totheark's message may be telling Jay to watch his back. Or that his death will soon come. Or that someone he knows will die

At 0:17, the text "HE IS SEARCHING" is overlaid with the further text "ALEX" and "FOR YOU", to make "ALEX IS SEARCHING FOR YOU".

This implies, once again, that Alex is not totheark but is being watched by totheark.

It could also make "He is searching for you Alex,' still implying Alex isn't totheark, but is being chased by the Operator, or even Masky.

Further Speculation

This is the first visual sign we've had of Jessica in some time. Maybe she's back in play one way or another, or maybe TTA is taunting Jay with another person he left behind. -JKat

One of the comments pointed out that at 0:32, you can hear something that vaguely sounds like "You shouldn't be in here", possibly relating to Alex's phone call in Entry 64. ~travestyoftheweek

At about 0:12, the words "I saw" overlay "You abandoned him". This implies that TTA was near the tunnel, watching Jay and Tim. Alex's phone call implies that he was as well. There is still a possibility that Alex is TTA, as they both knew what was going on, and were likely watching the events. ~RvB_Sigma

The first three seconds of the video show a riverbed and near the end of the video, the tunnel overlays Jessica's face, and the former is in turn covered by water. This could be TTA attempting to guilt-trip Jay over Jessica disappearing, and abandoning Tim. ~Dr SuperGreg